Conventionally, a DRM (Dipole Ring Magnet) type plasma processing apparatus has been utilized as an apparatus for performing a plasma process such as a film forming or etching process on a semiconductor wafer. The DRM type plasma processing apparatus includes a processing chamber for accommodating a semiconductor wafer therein and a plurality of cylindrical magnets that are arranged upright in a loop around the processing chamber. Each magnet is paired with another magnet which is arranged to have point symmetry about the central axis of the chamber with respect to each other. The magnets are connected to a rotation driving mechanism such that they all rotate synchronously. When each magnet (segment magnet) is rotated by 180 degrees, the magnetization direction makes one revolution (reversed). Further, in the DRM type plasma processing apparatus, a plurality of the cylindrical magnets are synchronously rotated to produce a horizontal magnetic field in the processing chamber, whereby a plasma process is performed on a semiconductor wafer which is horizontally disposed on a mounting table in the processing chamber (for example, see Reference 1). Such a DRM type plasma processing apparatus is appropriate for producing a unidirectional magnetic field in a large space. Further, compared to a plasma processing apparatus using a superconducting coil or a resistive coil, the DRM type plasma processing apparatus is relatively maintenance-free and easy to control the uniformity and directionality (skew angle) of a produced magnetic field. Therefore, it has been utilized as an external magnetic field producing unit in various semiconductor manufacturing processes.
The rotation driving mechanism has a motor, and the rotational power of the motor is transferred to respective cylindrical magnets to rotate them. In the DRM type plasma processing apparatus, the plurality of cylindrical magnets need to be rotated precisely to ensure a proper plasma processing on a semiconductor wafer. Accordingly, a conventional DRM type plasma processing apparatus includes a rotation detection unit (trigger sensor) that generates a rotation alarm when the detection unit is unable to detect any rotation of the cylindrical magnets. Also included is a motor abnormality detection unit that generates a motor alarm when any abnormality in the motor has been detected.
[Reference 1] Japanese Patent Laid-open Publication No. H7-130495
However, for example, even if the cylindrical magnets can be rotated, if their rotational velocity is not stable due to breakdown, wear and tear etc. of the rotation driving mechanism, the magnetic field produced on the semiconductor wafer in the processing chamber will become nonuniform. As a result, the plasma process cannot be performed stably on the surface of the semiconductor wafer. Ultimately, the overall in-surface uniformity of the plasma processing cannot be guaranteed; for example, on the surface of a single semiconductor wafer, spots in OPEN condition (excessive etching) and SHORT condition (insufficient etching) will result during etch processing.
Generally speaking, the maintenance cycle of a driving mechanism needs to be shortened to avoid its breakdowns and wear and tear, but such measures will compromise the steady and continuous operation of the DRM type plasma processing apparatus.